Fix #160: Anchor IsJSON regex to prevent false positives on CDATA

[bukzor]

Summary

Fixes part of issue #160 by anchoring the IsJSON() regex to prevent false positives when detecting format.

Problem: The IsJSON() function uses an unanchored regex \s*[{\[] which matches a [ character anywhere in the input. This causes XML documents containing CDATA sections like <![CDATA[text]]> to be incorrectly detected as JSON format.

Solution: Changed the regex to ^\s*[{\[] to anchor it to the start of the input string. Now only inputs that actually begin with { or [ (after optional whitespace) will be detected as JSON.

Changes

• internal/utils/utils.go: Modified IsJSON() to use anchored regex ^\s*[{\[]
• cmd/root_test.go: Added TestFormatDetection test to verify CDATA sections are correctly detected as XML format

Test Plan

• Added test case TestFormatDetection/CDATA_should_be_detected_as_XML_not_JSON
• Verified test fails without the fix (shows CDATA detected as JSON)
• Verified test passes with the fix (CDATA now correctly detected as XML)
• All existing unit tests pass (go test ./...)

Additional Context

This is part 2 of a 4-part fix for issue #160. The other PRs address:

1. HTML text escaping (PR Fix #160: Add proper text escaping in FormatHtml #161 - already submitted)
2. IsJSON regex anchor (this PR)
3. CDATA support in NodeToJSON
4. Exhaustive switch statements for node types

🤖 Generated with Claude Code

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.57%. Comparing base (ec5a59a) to head (9a5c191).
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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #163   +/-   ##
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  Coverage   80.57%   80.57%           
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  Files           5        5           
  Lines         690      690           
=======================================
  Hits          556      556           
  Misses         92       92           
  Partials       42       42           

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